It is possible to cash a check at the post office, but doing so is only available for very specific types of checks. The United States Postal Service cashes Postal Service Commercial Checks and Treasury Checks. The checks must not be more than a single year old.
There are two additional items that customers can cash at a United States post office. The first is domestic postal money orders, which customers can cash regardless of the date it was issued. The second is any other postal money order, such as an international money order. Customers must cash these before the expiration date.